Refrigerated spaces such as refrigerated display cases, walk-in refrigerators, and walk-in freezers commonly include heaters to prevent condensation from forming on certain areas of the device from water vapor present as humidity in the surrounding air. For example, walk-in refrigerators and freezers typically employ a heater to prevent condensation from forming on air vents, personnel doors, drain lines, and observation windows. Similarly, refrigerated display cases such as coffin cases, island cases, and tub cases typically employ a heater to prevent condensation from forming on and around an opening and/or door of the display case.
For example, glass-door refrigerated display cases are frequently used in supermarkets and convenience stores and often include heaters in the glass doors and the door frames to prevent condensation on the glass from humid air. The glass doors and frames are typically heated to a temperature above the dew-point temperature of the air in the room in which the display cases are located to prevent condensation.
Prior art control systems apply heat to the glass doors in proportion to a measured dew point in an open-loop system. Manual intervention, in the form of manually adjusting the control scheme, is required to achieve condensation-free doors. The adjustment process is prone to human error, typically resulting in setting the heat too high and losing some of the promised energy savings. Also, such adjustments usually are made at a particular operating condition, and may not work correctly year round where climate changes are more drastic, as dew point and conditions change with the season. Further, the adjustment process is time consuming and does not result in a known door temperature.
One method of controlling the amount of heat applied to the display case doors includes applying full power (i.e., line voltage, typically) to the door heaters. The applied heat prevents condensation but wastes energy as more heat is applied than is necessary. The excess energy consumed by the door heaters directly increases the cost of operating the refrigeration system. Such costs are further increased as excess energy in the form of heat is dissipated into the refrigerated space and must be removed by the refrigeration system.
Other control systems modulate the heat applied to the display case doors and, as a result, reduce door heat energy and related costs. Such systems generally control the applied proportion of maximum heat, which is proportional to the square of line voltage to adjust the heat applied to the doors. While such systems adequately reduce the amount of heat applied to the doors, such systems suffer from the disadvantage of being susceptible to variations in line voltage and are therefore not precise.
For example, as illustrated in FIG. 1, a prior art proportional controller has one or more adjustments to allow a user to adjust a door heater between a minimum and a maximum in response to variation of dew point of the room air (i.e., more heat for higher dew point). Some systems permit limiting the upper and lower limits of the heat modulation to values other than zero and one hundred percent, e.g., limiting the heat to a twenty percent minimum and a ninety percent maximum. Others have a simple rotary dial that adjusts a gain or an offset. Still others define limits as endpoints of a line, as illustrated in FIG. 2, which shows control over a 3-segment line. Segment 1, which is at a low dew point, shows modulation held at twenty percent of full heat. In segment 2, modulation varies with dew points between 25 and fifty degrees F. dew point. In segment 3, modulation is ninety percent, of full heat, for high dew points.